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Evan
12-22-2008, 06:01 PM
Somewhen I plan to do some aluminum casting so I have been playing with CNC foam cutting. It seems to have some promise for making complex shapes that would be difficult to machine.

http://metalshopborealis.ca/pics5/foam1.jpg

http://metalshopborealis.ca/pics5/foam2.jpg

Liger Zero
12-22-2008, 06:10 PM
Ok what sort of foam is that? Rigid PS foam?

Can you explain your setup there a bit more? Is the wire a "hot-wire" or are you just relying on tension to cut?

Evan
12-22-2008, 06:16 PM
That is rigid polystyrene and the wire is a hot wire. It's a very small guage nichrome running from a 6 vac transformer with a lamp dimmer in a box to vary the output. The spring on the cutter maintains constant tension on the wire as it will stretch considerably when it warms up.

Fasttrack
12-22-2008, 06:16 PM
Looks like a hotwire setup. Pretty slick! I would not have thought to do that. Seems like it would be difficult to figure out the programming for that just because I'm so used to thinking in terms of material removal, not actually cutting the part out. (if that makes any sense...)

Evan
12-22-2008, 06:24 PM
I made the part in CamBam but had to do some fancy footwork to get it to work. I first laid out the design in the X-Y plane. However, if you try to rotate a curve off the xy plane CamBam chokes even if you convert it to a polyline first. I found a workaround by distributing a bunch of points around the pattern and then converting them to a polyline. For some reason that can be rotated into the X-Z plane with no trouble. Then all I do is make it an engraving toolpath and it thinks it is engraving something on a vertical surface.

If you want the G-code for the biohazard symbol you can download it here.

http://metalshopborealis.ca/misc/biohazard.txt

It runs in the X-Z plane with no Y movement. Origin is at the top center of the shape and it makes the cut in one continuous move. When finished it retracts to the origin through the original cut it made to get to the starting point.

uute
12-22-2008, 08:12 PM
I like that tension arm, Evan. I've seen many that just didn't look very sturdy or reliable. Yours is well engineered as usual.
uute

S_J_H
12-22-2008, 08:23 PM
wow!!
That is really neat.

davidh
12-22-2008, 10:08 PM
give it a couple glued on sprues, coat it with drywall mud and drop it in a 5 gal pail of clean fine grain sand.

melt aload of aluminum and pour fast. it will surprise the heck out of you how easy and neat it will come out.

<a href="http://s260.photobucket.com/albums/ii12/tooldoc/?action=view&current=61.jpg" target="_blank"><img src="http://i260.photobucket.com/albums/ii12/tooldoc/61.jpg" border="0" alt="foam deer and sand scoop"></a>

<a href="http://s260.photobucket.com/albums/ii12/tooldoc/?action=view&current=61.jpg" target="_blank"><img src="http://i260.photobucket.com/albums/ii12/tooldoc/61.jpg" border="0" alt="foam deer and sand scoop"></a>

davidh
12-22-2008, 10:14 PM
try again. i did one in brass too, just to see if it would work. . .
http://i260.photobucket.com/albums/ii12/tooldoc/49.jpg

http://i260.photobucket.com/albums/ii12/tooldoc/78brassaluminum.jpg

DICKEYBIRD
12-22-2008, 11:28 PM
Now you're getting into yet another area of "better living through the wonderful world of computer control."

Model aircraft CNC foam wing cutting machinery/software is really well developed these days. The wire heat is handled by a PWM board that is controlled from within the software. As you've no doubt found, (and if memory serves, you have a history with model aircraft and already knew it) PS foam responds extremely well to the proper wire heat & feed rate; just like machining other materials. Too much heat and things go to crap...quick. Here's a couple links:
http://tech.groups.yahoo.com/group/CNCFoamcutters/
http://selectfreeware.com/node/269

Another fantastic example of the flexibility of your CNC mill. Inspiring work...I can only dream.:)

Your Old Dog
12-23-2008, 06:39 AM
A lot of the fancy chrome and stainless signage you see is done over that same foam. Figure out how to hard coat it like epoxy paint or something and you can be doing high priced desk name plates!

Evan
12-23-2008, 06:53 AM
I have done a lot of foam cutting in the past for models. I have a large cutting frame and a small hot wire jig saw that I built way back from scraps. It still works, I think.

I have been experimenting with the wire temp vs cutting speed. The real limiting factor in foam cutting is wire drag. If you cut too fast for the wire temp it drags back and distorts the shape. If you tighten it to prevent that then it may break. If you slow down too much the wire stays too hot and makes a much wider kerf.

Feeds and wire temperature ar much fussier than feeds and speeds for metal cutting. You have to find the precise balance of the variables that will produce the best result. It usually requires some empirical testing with the particular shape being cut.

Peter.
12-23-2008, 07:40 AM
I've also cut a few clarke-y's using a 3 foot wire and I found that the wire was hotter in the middle than at the ends, causing either a wider cut if you went slow enough not to drag or a distorted shape too. Invariably I had to tidy it up with sandpaper. For a PSU I used a gutted-out CBradio mains transformer.

Evan
12-23-2008, 10:05 AM
Here you can see what a difference changing the cutting speed and the temperature makes. The first one is on the left.

http://metalshopborealis.ca/pics5/foam4.jpg

alanganes
12-23-2008, 10:39 AM
give it a couple glued on sprues, coat it with drywall mud and drop it in a 5 gal pail of clean fine grain sand.

melt aload of aluminum and pour fast. it will surprise the heck out of you how easy and neat it will come out.


David,
Is that thing with the drywall mud your idea, or did you see that someplace else? Looks amazingly simple and effective. Would you care to elaborate on that a bit or point to additional info?

I don't want to hijack the thread, please feel free to start a new one. ;)

Thanks,
Al

Evan
12-23-2008, 11:31 AM
No hijack. It's called lost foam casting. You coat the foam with several layers of plaster and dry it completely. That is what I have in mind to make parts that just aren't practical to make otherwise.

rotate
12-23-2008, 12:21 PM
I have tried using plaster of paris and joining compound, but found that it can't stand up to the heat.

Has anyone tried using high temperature motar, the kind used for bonding fire bricks?

Evan
12-23-2008, 12:39 PM
Have you tried adding anything to the plaster? I was thinking something like silica sand or maybe chopped mineral wool.

Mcgyver
12-23-2008, 01:11 PM
Evan, you sure are getting you money's worth out of the mill! good work.

there's a ton of lost foam info as well as plaster coating ideas here, really neat site worth checking out if you haven't seen it before

http://www.buildyouridea.com/

John Stevenson
12-23-2008, 01:48 PM
I have tried using plaster of paris and joining compound, but found that it can't stand up to the heat.

Has anyone tried using high temperature motar, the kind used for bonding fire bricks?

You need Plaster of Paris from France, not Paris, TX.
...................:rolleyes:

.

Duffy
12-23-2008, 02:44 PM
Evan:- Have you given any thought to or tried using electrostatic precipitator wires instead of nichrome? I salvaged the wires, which I believe are tungsten, from a couple of Honeywell units, and would be glad to send you a couple to try if it is worth it. They are only abot 0.001" with a small grommet swaged on each end. I have yet to try hotwire cutting.

derekm
12-23-2008, 02:57 PM
Evan:- Have you given any thought to or tried using electrostatic precipitator wires instead of nichrome? I salvaged the wires, which I believe are tungsten, from a couple of Honeywell units, and would be glad to send you a couple to try if it is worth it. They are only abot 0.001" with a small grommet swaged on each end. I have yet to try hotwire cutting.

0.001" wire :eek: What sized precips were those?
I have two precip wires from a Flakt (formerly Svenska Flaktfabriken) precip which are a coiled wire thats 3mm thick with a large hook on each end. When stretched these are about 12m long. These were probably from either a cement or Gypsum works...

dp
12-23-2008, 03:06 PM
I have been experimenting with the wire temp vs cutting speed. The real limiting factor in foam cutting is wire drag. If you cut too fast for the wire temp it drags back and distorts the shape. If you tighten it to prevent that then it may break. If you slow down too much the wire stays too hot and makes a much wider kerf.

This presents an interesting control problem - trying to maintain constant temperature where the wire resistance is subject to variations caused by both load (heat sinking) and current. I don't recall the temp coefficient of nichrome but it's got to be a factor.

Evan
12-23-2008, 03:19 PM
Part of the problem is time lag. The wire doesn't heat up instantly. You can use a thick wire with some thermal mass and that helps the moment to moment variation problem but at the expense of a much larger kerf. I haven't tried using a constant voltage source but that should help a lot. If it cools off you want it to heat back up as fast as possible.


Thanks Duffy,
I have a lot of .001 tungsten wire that I can play with as well as platinum, gold and nickel wire in the same size range.

noah katz
12-23-2008, 03:53 PM
"This presents an interesting control problem - trying to maintain constant temperature where the wire resistance is subject to variations caused by both load (heat sinking) and current. I don't recall the temp coefficient of nichrome but it's got to be a factor."

I had a '79 BMW with a mass airflow sensor that worked measuring by how much current it took to maintain a wire at constant temp; I think that's called a hot wire anemometer.

Yep; lots of google hits.

I wonder how you measure the temp of the wire w/o putting a lump in it.

dp
12-23-2008, 04:03 PM
I wonder how you measure the temp of the wire w/o putting a lump in it.

It's resistance is related to temperature but also strain. Strain is related to load. The length also changes with temperature and load. Load is related to foam density and speed of cut which also affects temperature :)

It's quite a complex model.

winchman
12-23-2008, 04:05 PM
When you run out of things to cast, try one of these:
http://blog.makezine.com/i1536-2442-4-21-1-f01.jpg

You don't even have to prep the mold.

Roger

darryl
12-23-2008, 04:37 PM
I've had better luck by staying away from the really thin guages of element wire. The 'brute force' approach seems to work best for me, and that's using heavier wire, more current, and tighter voltage control. I accept the wider kerf. My power supply is a transformer with a lot of taps on the secondary. The first 4 or 5 taps are one at every secondary turn, then the rest are about 4 turns apart. Then there's a separate secondary winding that can be added in by using a jumper. There's about 20 terminals in all, and the transformer is capable of a lot more current than the hot wire requires. This gives it the capability of maintaining a low voltage drop under load.

The first 5 taps are used for fine voltage control, and the others are selected to give the bulk of the voltage required by the particular hot wire length being used. For the longest wire, which in my case happens to be about 50 inches (for slicing 4 ft wide sheets into thinner sheets) I add in the separate secondary winding.

I used to use a variac running a separate transformer, but I found the variations to be a bit much. I'm happier with the crude transformer with multiple taps method of heat control.

One notable benefit of using heavier gauge wire is that the ends of the cut where the wire exits the foam don't shrink away so much from excess heat, as happens when thinner wire is used and the exposed ends are free to heat a lot more than the wire still engaged in the foam. You can power the wire, set the heat to a barely visible dull glow, then plunge it into foam and watch the exposed ends begin to glow much more brightly. Because the heat is concentrated at these points, it will shrink the foam away at the sides. Heavier wire minimizes ths, as does using a wire that's much longer than needed for the particular cut. I would recommend though that at least 3/4 of the length of heating wire should be in the foam, or conversely the wire should not be more than 3/4 of an inch exposed out each side of the foam.

I used a fairly thin wire to slice the 4x8 sheets into 1/8 thick slabs, but it wandered a lot, even though I had an automatic tensioning setup in use. These slabs were to be used to make a helium filled blimp, but that project got nixed- I wish the guy would have finished it. It was his school project, not mine, and the task was falling into my hands to complete- well you know how that goes.

Now that reminds me of one of my own projects which I never completed, the flying saucer. This one is 4 ft diameter and 6 inches thick top to bottom, of course hot-wired to shape and hollowed out a lot. That could be just the model to use a co2 motor on- hmm. Another shop project in itself.

Be careful to not breath the styrofoam fumes-

alanganes
12-23-2008, 04:58 PM
Have you tried adding anything to the plaster? I was thinking something like silica sand or maybe chopped mineral wool.

I've seen lost foam casting before, what I had not seen was anyone using drywall mud in sand like that. Pretty cool. I really like the quick-and-dirty-ness of it. Might have to try that when springtime arrives.

As to additives to the plaster. I also seem to vaguely recall reading something on-line about someone using a disk in a drillpress to "whip" lots of air into a batch of plaster for use as a poor man's investment. The theory was that the air bubbles would give the plaster enough additional insulating properties for it to hold up for the one-time use. This goes back quite a few years, so I may have the some of the details way off. I don't recall how successful this all was. Food for thought, maybe.

Evan
12-23-2008, 05:07 PM
Hmm. Could whip up a batch of egg whites and fold them in to make a plaster mousse. It should work as egg whites are pure protein, no fats, and will dry out nicely. Good glue too.

Or, maybe use beer instead of water to make the plaster?

Astronowanabe
12-23-2008, 10:20 PM
You need Plaster of Paris from France, not Paris, TX.
...................:rolleyes:

.


metric plaster?

Astronowanabe
12-23-2008, 10:28 PM
I bet you could hook the wire tensioner up so that increased pressure
increased the current. so that speeding up the wire made it hotter and slowing down cooled it. once calibrated and tuned it might hep get the best finish on any shape

Circlip
12-24-2008, 06:24 AM
Wires I eventually graduated to are 0.5 - 0.7mm dia Piano wire. Don't stretch like nichrome, just have to fiddle with the electrics. Using a fireclay slurry and letting it dry outish between coatings is Investment Casting.

Regards Ian.

GKman
12-24-2008, 07:44 AM
You need Plaster of Paris from France, not Paris, TX.
...................:rolleyes:

.

Is that like, you know, Paris Hilton plastered?

alanganes
12-24-2008, 08:08 AM
Hmm. Could whip up a batch of egg whites and fold them in to make a plaster mousse. It should work as egg whites are pure protein, no fats, and will dry out nicely. Good glue too.

Or, maybe use beer instead of water to make the plaster?


It's hard to argue with a good mousse. Besides, I've known people to do things at least that odd, except it was generally after they had consumed the beer, rather than adding it to the plaster...

Evan
12-24-2008, 08:58 AM
Aluminum melts at 1220F. That's nothing in the ceramics world, about cone 19. All it should take to make a protective coating is a slip coat of earthenware clay followed by plaster.

laddy
12-24-2008, 09:24 AM
We used to and still use a product called Gray-Pak by Ransom & Randolph co. of Toledo Ohio for investing gold where one piece needed to be soldered to another without melting either casting in the process. It was supplied as a flour-like powder mixed with water to form a very smooth creamey refractory reinforced plaster of paris. Dried quickly and always gave good results. Should work for Aluminum as well Fred

A.K. Boomer
12-24-2008, 10:52 AM
I have done a lot of foam cutting in the past for models. I have a large cutting frame and a small hot wire jig saw that I built way back from scraps. It still works, I think.

I have been experimenting with the wire temp vs cutting speed. The real limiting factor in foam cutting is wire drag. If you cut too fast for the wire temp it drags back and distorts the shape. If you tighten it to prevent that then it may break. If you slow down too much the wire stays too hot and makes a much wider kerf.

Feeds and wire temperature ar much fussier than feeds and speeds for metal cutting. You have to find the precise balance of the variables that will produce the best result. It usually requires some empirical testing with the particular shape being cut.



My knowledge of electrics and electronics is just enough to make me dangerous -- But --- if my rusty archives hold true doesn't resistance change in most wires being utilized for thermal purposes - or at the very least I know that that holds true for certain types, then with this being the case one should be able to install an in line sensor that picks this reading up and adapts the power thats feeding the wire to suite the immediate demand, foam is a "cooler" on the element as it dissipates the elements energies and therefore would make the entire wire temp/feed pretty tricky like you state, but incorporate a simple automatic adjustable voltage/amperage rate and it would really create a much easier situation to deal with -- to some "degree" it may be applicable to achieve this somewhat automatically within the choice of cutting wire itself - a sort of self regulation so to speak - but not as good as with a variable volt/amp control... ?
Experimenting with either a variable caliper type element adjuster for the specific foam your working on OR blasting air by the exposed wire (so faster speeds can be used without burning out the element) could also open up new avenue's...